A gun barrel's critical dimensions, although there are corresponding standard internal diameters depending on gauge numbers, differ in actual internal diameter depending on the manufacturer. The critical dimensions of the gun barrel are: the bore; choke; chamber length and diameter, forcing cone length and barrel wall thickness. These dimensions are important to know by the gun user for safety reasons and their effect on performance and accuracy of the gun. Specifically, the chamber length is important to use the ammunition for which it was intended. Knowing the dimensions of the choke is critical for the game or target on hand as the choke of a gun, or the decrease in diameter of the bore near the end of the barrel, controls the shot pattern. It is critical to know the amount of wear and degree of useful life of a barrel for safety reasons determined by the barrel wall thickness. Likewise, by knowing these critical dimensions, the user can determine if the chokes on a gun have been altered from the barrel markings, or if the barrels are original or after market or have been overbored, or if the chamber has been altered or reproofed.
Inventions have been made for measuring the internal diameter of a gun barrel. U.S. Pat. No. 4,348,812 describes an elongated tool inserted into a gun barrel having blades that expand outward when a rod member is slide forward. The blades engage with the inner surface of the gun barrel and the diameter is read from the indicia on the rod. Also, a set point member can be locked on the indicia marking and the user removes the tool from the gun barrel by retracting the rod to cause the blades to move back together. Then the arms are reset outside the gun using the set point position and the diameter may be measured with a micrometer.
Other inventions utilize a scissors mechanism which expand when inserted in a gun barrel and then withdrawn and the degree of expansion is measured using a micrometer.
Another use of the preferred embodiment of the present invention is for use in measuring bore diameters in any tube, pipe or cylinder used in an industrial application.
There is a continuing need for a relatively inexpensive device in which the measuring probes are expanded by a spring force eliminating any human intervention and the diameter of expansion measured while engaged within the barrel without the need to use a micrometer.
There is also a continuing need to provide a device that can easily and quickly be recalibrated to ensure repeatable accurate readings.